One approach to preparing photographic dispersions containing polymer is to load a hydrophobic photographically useful compound into a polymer latex. Manufacturing advantages of loaded latex dispersions can include avoiding the high-shear or turbulent mixing required to prepare conventional emulsified photographic dispersions, and the reduction or elimination of high-boiling solvents, known as coupler solvents. Also, photographic advantages of polymer-containing photographic dispersions can be obtained with loaded latex compositions, including improved image permanence, improved dye hue and color reproduction, and improved dry and wet film physical properties.
The use of latex or dispersed polymers in the preparation of photographic dispersions has been described in U.S. Pat. Nos. 2,772,163; 2,852,382; 4,133,687; 4,199,363; 4,203,716; 4,214,047; 4,247,627; 4,368,258; 4,448,850; 4,497,929; 4,608,424; 4,684,608; 4,724,197; 4,822,728; 4,840,885; 4,891,309; 4,914,005; 4,990,435; 5,026,631; 5,047,316; 5,091,296; 5,279,931; British Patent GB 1,287,013; Canadian Patent No. 616,178; European Patent Application EP 483,416; Japanese patent application JP 032 558; and Research Disclosure 181,072. Usually these latex polymers are prepared by emulsion polymerization, although emulsified dispersions of organic-soluble polymers are also described, as in U.S. Pat. Nos. 4,388,403; 4,840,885 and 5,026,631.
The usual procedure for preparing a loaded latex described in the prior art is to combine a solution of the hydrophobic photographically useful compound in a water-miscible organic solvent with the aqueous latex. The resulting mixture that typically has about a 1:1 ratio of water to organic solvent, is either diluted with water or the organic solvent is removed by evaporation, with the result that the hydrophobic compound becomes associated with or dissolved in the latex particles. Variations on this procedure vary the order of addition of the organic solution and aqueous latex, substitute volatile, but not entirely water-immiscible auxiliary solvents for the water-miscible auxiliary solvents, incorporate water-miscible or volatile organic solvent in the emulsion polymerization step that is also present during dispersion preparation, or require the formation of intermediate water-in-oil emulsions of the latex in volatile organic solvent before the formation of the final oil-in-water loaded latex dispersion. In some cases, photographically useful compounds are dissolved in the organic monomers prior to emulsion polymerization. Procedures are also described in which base-ionizable couplers and/or base-ionizable latex polymers are combined at high pH, often with auxiliary solvent present, followed by neutralization and/or addition of magnesium salts or alkaline-earth metal salts, to form a dispersion of coupler and polymer.
All of these procedures for preparing loaded-latex or latex-containing dispersions present severe practical difficulties. Rigid requirements exist for both the hydrophobic compound to be loaded and the latex, especially for the procedures that use water-miscible organic solvent. In the initial mixture of hydrophobic compound, water-miscible organic solvent, and latex, the hydrophobic compound must not be precipitated by the aqueous environment, and the latex must not be coagulated by the large amount of organic solvent present. Many patents in the prior art describe a test for latex loadability, in which a suitable latex must not coagulate when mixed with an equal volume of the water-miscible organic solvent used in the dispersion preparation. Most latex polymers do not meet this requirement. A second difficulty is that auxiliary solvent is used in the process at all, causing severe manufacturing, environmental and safety problems. A third concern is that free-radical emulsion polymerization of monomers with photographically useful compounds dissolved in the monomers can cause chemical destruction of the compounds and can impair the polymerization process, leading to unwanted crosslinking, or lowered polymer molecular weight, and to higher levels of residual monomer. Polymerization processes other than free-radical polymerizations, including most condensation polymerizations, are poorly adapted to production of emulsion polymers, and also present similar difficulties with unwanted reactions of the photographically useful compounds under polymerization conditions or with the polymerization reagents, and unwanted effects of the compounds on the polymerization process, including chain termination or crosslinking. None of this prior art describes procedures for loading latex polymers without the use of substantial amounts of water-miscible or volatile auxiliary solvent at some point in the procedure. A fourth problem is that it is often difficult or impossible to achieve high loading levels, i.e., greater than about a 1:1 ratio, of the hydrophobic compound or compounds in the latex, using the known methods.
We have recently discovered that subjecting a mixture liquid oil-phase and an aqueous polymer latex to conditions of high shear and/or turbulence can lead to formation of loaded latex compositions, even in the absence of water-miscible or volatile organic solvent. This method is applicable to a wider variety of latex polymers and hydrophobic photographically useful compounds than the other methods described above, and higher loading levels can be achieved. This method, however, still requires the energy to cause high-shear and/or turbulent mixing of the dispersion containing the latex and the photographically useful compound.